1. Field of the Invention
The present invention relates to a coating film forming apparatus for forming a coating film on a substrate such as a semiconductor wafer.
2. Description of the Related Art
In a manufacturing process for a semiconductor device, a liquid crystal display device and an LED, a step is performed which forms a coating film by applying a coating solution such as a resist solution, a conductive paste onto a substrate.
As a method of forming the coating film on the substrate, there is a known spin-coat method of dripping the coating solution from a nozzle to a central portion of the substrate with the substrate is being rotated and diffusing the coating solution on the substrate by the centrifugal force. However, in the spin-coat method, the dripped coating solution is applied and spread by the centrifugal force and therefore the most of the coating solution is scattered from an outer peripheral edge portion of the substrate and is discarded. Accordingly, the spin-coat method is a method not adequate to the case of forming the coating film using, for example, an expensive material such as a conductive paste containing rare metal fine particles.
As another method of forming the coating film on the substrate, there is a known slit-coat method of forming the coating film by relatively moving a nozzle and a substrate with a solution puddle formed between a nozzle having an elongated discharge port in a slit shape and a substrate (FIG. 1 of Japanese Laid-Open Patent Publication No. 2004-114012, FIG. 3 of Japanese Laid-Open Patent Publication No. H8-138998).
However, the coating film formed by the slit-coat method has a problem of being likely to be uneven in film thickness within the substrate. Hence, it is suggested to regulate the pressure inside a storage chamber that stores the coating solution in the nozzle so as to make the discharge amount of the coating solution from the discharge port constant in a coating film forming apparatus by the slit-coat method (What is claimed is of Japanese Laid-Open Patent Publication No. 2013-98371).
In the slit-coat method, the coating solution is discharged utilizing the own weight of the coating solution in the nozzle and the decrease in surface tension of the coating solution by the coating solution exposed to the discharge port of the nozzle adhering to the substrate. From the principle, the coating film formation by the slit-coat method is performed under assumption of treating a substrate having a width equal to or smaller than the length of the elongated discharge port. Further, in the slit-coat method, the discharge width of the coating solution to be applied onto the substrate is almost equal to the width of the substrate. Note that the width of the substrate means the length in a direction perpendicular to the relative movement of the nozzle and the substrate.
The width of the substrate is constant, for example, in a rectangular glass substrate for FPD. However, for example, in the semiconductor wafer forming a circle, the width of the substrate gradually changes in a process of relatively moving the nozzle and the substrate. As described above, in the slit-coat method, the discharge width of the coating solution to be applied onto the substrate is almost equal to the width of the substrate. Accordingly, when the width of the substrate changes, the discharge width of the coating solution also changes. Therefore, when the discharge pressure inside the nozzle is constant, the coating solution to be discharged from the discharge port tends to be insufficient in a process of the width of the substrate increasing, conversely, the coating solution to be discharged from the discharge port tends to be excessive in a process of the width of the substrate decreasing, during the coating. This results in a problem in which in the case of treating the substrate changing in width such as a semiconductor wafer, the film thickness is small in the first half of the coating and is large in the second half of the coating near the center of one substrate as a boundary, causing a difference in film thickness of the toting film within the substrate.